1. Filed of the invention
The present invention relates to constitution of a photoconductor, particularly constitution of a photoconductor which enables to improve its response velocity.
2. Description of the prior art
An example of prior art photoconductors is shown in attached FIG. 3. Such photoconductor has already been reported, for example, by Sze in "Physics of Semiconductor Devices" (1981) at pages 744-748. The photoconductor of FIG. 3 comprises a semi-insulating GaAs substrate 1, an active layer 33 consisting of non-doped GaAs formed on the layer 1 and ohmic electrodes A1 and A2 formed on opposite ends of the active layer 33. When a light beam 4 having energy exceeding a bandgap of GaAs is impinged on the upper surface of the active layer 33, under condition that an electric potential is loaded between the electrodes A1 and A2, electron-hole pairs are generated to cause flow of photocurrent.
The photocurrent gain GN and the response time TR of a photoconductor can be expressed by the formulae as follows: EQU GN=Un.multidot.Tc.multidot.E/L (1) EQU TR=L/(Un.multidot.E) (2)
wherein Un is a carrier mobility, Tc is a carrier lifetime, E is an electric field and L is an electrode spacing. In a photoconductor whose active layer is formed by a non-doped semiconductor, when compared with a photodiode or the like, a greater gain can be obtained owing to a longer carrier lifetime, but on the other hand, a larger electrode spacing and a lower electric field are employed and so there is a problem of a longer response time.